Cylindrical cells, mostly alkaline cells are widely used. These cells are comprised of an elongated cylinder constituted by a metal can with press fitted cathode pellet rings containing manganese dioxide as the active electrode material in the interior of the can to constitute the positive cell electrode. An anode gel composed of zinc powder active material, gelling agent and an alkaline electrolyte filling the cylindrical central cavity of the positive cell electrode so that a cylindrical separator made of a specific sheet material separates the two electrodes. The separator must be composed of a material that allows ions to pass from one electrode to the other, but prevent particles of the two electrode materials from passing through, and also be an electrical insulator to prevent electrons from passing directly though. The active area of the separator is the portion where it directly separates active cathode material from anode material. A critical portion of the separator is the central bottom part, because the anode gel expands significantly during discharge of the cell and the bottom part has to remain intact separating the anode from the interior of the can, which would cause an internal short circuit and cell failure.
A conventional way of providing a reliable insulation is described in U.S. Pat. No. 6,099,987 wherein an outer and an inner isolating cup are attached to the lower end of the separator, and the interior bottom part of the separator is sealed by the application of a thermoplastic sealant. This is a perfect solution as far as isolation of the two electrodes are concerned but the presence of the cups and of the sealant takes a substantial cell volume, which cannot be utilized for cell function and requires the handling of several separate material parts.
U.S. Pat. No. 6,541,152 shows a different design also utilizing an insulating cup at the bottom and it has the same problem of decreasing useful cell volume and requires the handling of two separate material parts.
U.S. Pat. No. 6,270,833 does not use any cup but the separator is made longer than the required useful length in the cell, the windings of the cylindrical separator body are bound together with a binder, and the extended portion is first pushed inwardly by a tool moving normal to the cylinder axis then folded back to close the initially open end. The folded and closed separator forms a self containing unit that should then be inserted into the cell. The smooth insertion requires a small clearance between the inner diameter of the cathode rings and the separator, which could increase cell resistance. The closing operation of the bottom part is complicated and requires movements in different directions, and problems can arise by the inevitable appearance of wrinkles.
U.S. Pat. No. 6,035,518 describes a different method of making the separator, in which the separator material is wound around a mandrel and the winding is kept on the mandrel by a vacuum, and the separator does not constitute a self-containing unit, it should be guided until insertion into the semi-finished cell, wherein the winding tries to open up and fill the whole available space. While the idea of guiding the separator until insertion into the cell is preferable, the key problem, i.e. the closing of the bottom is solved here by the application of a hot melt sealant to fill the cell bottom including the bottom region of the separator. The presence of a sealant at the active lower region of the separator also decreases the available useful cell volume.
A further problem characteristic to separators used for secondary cells lie in that often a laminated structure should be used, since in case of secondary cells a thin semi permeable membrane layer, such as a cellophane layer should be provided. Two or more layered laminates are expensive and adhesives used to affix the layers contribute to higher internal resistance.
There is a further issue concerning separators that concern the need of synchronization with the general cell manufacturing process. State of the art processes produce at high speeds of 600 to 1200 parts per minute, and this high speed favors or requires easy to use technologies that can fit into the manufacturing line, rather than preparation of off-line, pre-fabricated separators, which can cause problems from additional handling.